Recently, in response to rapidly increasing demand in communications, a wavelength division multiplexing (WDM) optical communication system which can expand communication capacity without laying more fibers by transmitting optical signals at different wavelengths through one fiber is being developed. A DFB quantum dot laser that can output a single wavelength is employed in the WDM optical communication system.
A quantum dot laser may be simply implemented by forming an electro-optic conversion layer of an active layer to have a zero-dimensional quantum dot structure instead of a conventional bulk or quantum well structure. However, most practical quantum dots are formed by self-assembly. Accordingly, when an active layer is formed using a quantum dot, power that is locally distorted due to a certain percentage of lattice mismatch is included in the active layer.
A conventional DFB quantum dot laser structure will now be described with reference to the drawings.
FIG. 1 is a partially cut-away perspective view illustrating a conventional DFB quantum dot laser structure. Referring to FIG. 1, the conventional DFB quantum dot laser structure 100 includes a pair of electrodes 110 and 190 disposed on its top and bottom, an active layer 150 including a plurality of quantum dots (not illustrated), and a diffraction grating 140 disposed below the active layer 150. Also, the DFB quantum dot laser structure 100 includes an InP substrate 120 disposed on the lower electrode 110, a first light guide 145 adjacent to the diffraction grating 140, a second light guide 155 disposed on the active layer 150, and a current blocking layer 160 disposed at both sides of the active layer 150, and the first and second light guides 145 and 155. The current blocking layer 160 includes p-In, n-InP, p-In. Also, the DFB quantum dot laser structure 100 further includes a clad layer 170 disposed on both the active layer 150 and the current blocking layer 160, and a cap layer 180 (an ohmic layer) disposed on the clad layer 170.
As shown in FIG. 1, when the diffraction grating 140 is disposed below the active layer 150 in order to produce a single light source, formation of a quantum dot constituting the active layer 150 may be adversely affected by a non-planarized surface due to unevenness of the diffraction grating 140.
To resolve this problem, a space layer comprising a binary compound, i.e., InP, may be deposited to a predetermined thickness prior to forming a quantum dot, so as to ensure a planarized surface. However, when the space layer is formed below the active layer, since an optical mode formed in the active layer is far from the diffraction grating in optical coupling, single mode purity may be reduced by a small optical coupling constant in spite of a sufficiently long length of a resonance layer.
FIG. 2 is a partially cut-away enlarged view of a metal diffraction grating formed on a substrate of a conventional quantum dot laser structure. The quantum dot laser structure 200 shown in FIG. 2 includes an active layer 250 disposed on a substrate (not illustrated), a plurality of metal diffraction gratings 260 disposed on the active layer 250, and a waveguide 270 disposed on the metal diffraction gratings 260. Here, the metal diffraction gratings 260 are vertically optically coupled to the top of the active layer 250, and the waveguide 270 is perpendicular to the metal diffraction gratings 260.
When an experiment for making a light source is performed with the quantum dot semiconductor laser structure 200 having such a structure, a high-purity single mode light source is made. However, to form the metal diffraction gratings 260 shown in FIG. 2, electron beam lithography should be used instead of conventional UV photo-lithography. Thus, production cost and time may increase, thereby decreasing mass-productivity.
Also, the quantum dot laser structures 100 and 200 shown in FIGS. 1 and 2 should employ a long resonator whose length reaches the millimeter level because of the small volume of a gain material compared to a semiconductor laser using a conventional quantum well structure. Accordingly, it may not be easy to stably obtain a single mode due to local carrier saturation and hole burning phenomena.
To overcome this problem, a quantum dot laser having a high quantum dot surface density, which is one of a GaAs series having a wavelength band centered at 1.3 μm or less and an InP series having a wavelength band centered at 1.55 μm, may be employed. The GaAs series quantum dot laser structure uses a resonator having a length of about 300 μm like the conventional quantum well structure, so the structure can show relatively stable optical characteristics. However, since the InP series quantum dot laser structure uses a long resonator whose length is 1 mm or more, it is not easy to stably implement an optical mode.
As another structure for resolving this problem, a multi-electrode structure in which a gain region is separated from an optical waveguide region or a phase controlling region has been suggested. However, in such a structure, the diffraction grating part should be separated, which complicates a manufacturing process and decreases yield. And, a current source should vary depending on region, which complicates module production and increases power consumption.